This invention is related to Radio Frequency (RF) shielding to reduce Electro-Magnetic Interference (EMI) caused by RF emissions. In particular the claimed invention simplifies the attachment of an RF shield fence to printed wiring boards.
RF circuits are well known in the art to produce, amplify and modulate radio frequency energy. RF energy radiating from RF circuits is also known to increase the noise level that, in turn, interferes with the operation of adjacent electronic circuits. For example, the operation of adjacent electronic circuits may become erratic and unpredictable as RF emissions, and the resultant noise level, ebb and flow from the RF circuits. This RF interference, also known as electromagnetic interference (EMI), is an extremely important issue in determining the proper performance and functionality of electronic circuits adjacent to RF circuits. Accordingly, it is necessary to reduce the interference caused by radiating RF energy to protect sensitive electronic components and limit the level of radiation emanating from a product.
One method to protect sensitive components from RF interference is to place RF shields around critical electronic components. An RF shield is a continuous conductive structure that surrounds electronic components to prevent RF radiation from entering, leaving, or passing through the shield. Metallic RF shields, commonly referred to as xe2x80x9cshield cans,xe2x80x9d separate RF circuits from one another, and provide isolation between signal paths, are well known in the art. RF shields are generally constructed in two parts; a thin continuous metallic wall that surrounds the circuitry and a cover that extends over the RF circuitry and attaches to the continuous metallic wall. The shield wall, commonly referred to as a fence, is typically soldered to a printed circuit board, or printed wiring board (PWB), and connected to the electrical ground of the board. The cover is held attached to the fence by numerous means that are well known in the art.
One well known method of attaching an RF shield to a PWB is by soldering the RF fence to the electrical ground of the PWB through tabs that reside on the bottom edge of the fence. Various means of attaching and holding metal tabs to PWBs are well known in the art; see U.S. Pat. No. 4,343,530 to Leger and U.S. Pat. No. 5,379,026 to Ohashi.
Tabs created on an RF shield fence are, typically, either rectangular or square in shape and require apertures in the PWB to be either slotted or circular in form. As the apertures created are generally larger than the tabs, the tabs fit loosely in the apertures and during the soldering process the thermal mismatch between the tabs and the PWB causes the tabs to lift out of the apertures. The lifting of the tab raises the RF fence off the surface of the PWB and creates a space, or gap, between the upper surface of the PWB and the fence. RF emissions are thus able to escape from, or enter into, the RF shield through this space, and the efficiency of the shield is diminished. Thus, there is a need to hold the RF shield fence in place during the solder process to prevent the fence from lifting off the surface of a PWB and the subsequent costly process of filling the gaps created through a hand soldering process.
The method of the invention forms elongated apertures that provide a snap fit attachment of square or rectangular tabs of RF shield fences to printed wiring boards. This snap fit attachment is produced by utilizing a unique methodology and does not require special tools or tooling. According to the method of the invention, a plurality of co-linear, intersecting, circular elements form an elongated aperture that securely holds a square or rectangular tab. At the intersection of the circular elements, prong-like protrusions extend into the created elongated aperture to decrease the internal dimension of the elongated aperture. The inserted tab is then held between these prongs as the fence is permanently attached to the PWB. The number and placement of a plurality of circular elements determines the lateral and internal dimensions of the resultant elongated aperture.
It is a advantage that the elongated apertures are formed, in accordance with the method of the invention, solely using circular openings, such as can be produced by a drill. By using a simple drilling operation, the cost of creating elongated apertures to hold square or rectangular tabs is reduced as neither special tooling nor milling is necessary.